Flowmeter for measuring the flow rate of a two phase fluid in a pipe

ABSTRACT

A flowmeter for measuring the flow rate of a two phase liquid in a pipe having a first pipe section (11) having a first diameter (D1), a second pipe section (12) of a smaller diameter (D2), an internally tapering transition pipe section (13) forming a smooth transition between the first and second pipe sections (11, 12), and three pressure tapping points comprising upstream (At u , Bt u , Ct u ), downstream (At d , Bt d , Ct d ) and intermediate tapping points (At i , Bt i , Ct i ), in which a first differential pressure monitoring arrangement is provided to measure the pressure differential between the upstream and the downstream tapping points and form which fluid density can be derived, and a second differential pressure monitoring arrangement is provided to monitor the pressure differential between the intermediate tapping point and one of the upstream or downstream tapping points, whereby flow rates can be calculated from the derived density value and the second differential pressure measurement.

This invention relates to a flowmeter for use in measuring the flow rateof a two phase fluid in a pipe, such as an oil production pipe.

The invention has been developed primarily in connection with themeasurement of the flow rate of fluids extracted from undergroundreservoirs of hydrocarbons, which can comprise liquid/liquid mixturesand gaseous/liquid mixtures, such as water/oil mixtures and naturalgas/hydrocarbon mixtures.

It is known e.g. from GB2186981 (Schlumberger Limited) to provide aflowmeter for measuring the flow rate of a two phase fluid and whichcomprises a gradiomanometer and a venturi meter in series and which areformed by a first section of pipe with a first diameter, a secondsection of pipe with a second diameter less than the first diameter, athird intermediate section of pipe connecting the first and second pipesections and having a diameter changing smoothly and gradually from thefirst diameter to the second diameter, and an arrangement of pressuresensors placed at selected positions in the pipe sections to monitorpressure differentials and from which the flow rate of the two phasefluid can be derived.

The arrangement of pressure sensors comprises a first pressure sensorpositioned in the neighbourhood of the start of the change of diameterof the third section of pipe from the first diameter to the seconddiameter, a second pressure sensor in the first section of pipe spacedupstream from the first pressure sensor, and a third pressure sensorspaced in the pipe downstream from the first pressure sensor. Means isprovided to make a first differential pressure measurement between thefirst and second pressure sensors which is indicative of the density ofthe fluid flowing through the pipe, and means is also provided formaking a second differential pressure measurement between the first andthird pressure sensors, and from this there can be derived the flow rateof the fluid flowing through the pipe from the density value obtainedfrom the first differential pressure measurement and also from thesecond differential pressure measurement which is obtained.

The Schlumberger reference also describes in detail the calculationsinvolved in determining the flow rate of the two phase fluid.

The present invention is concerned with an improved arrangement ofpressure sensors in a pipe through which a two phase fluid is flowingand is to be monitored, and improved means of deriving pressuredifferential measurements from which the flow rate of the fluid can bederived.

According to the invention there is provided a flowmeter for measuringthe flow rate of a two phase fluid in a pipe and which comprises a firstpipe section having a first diameter, a second pipe section of a smallerdiameter, an internally tapering third pipe section forming a smoothtransition between the first and second pipe sections, and threepressure tapping points comprising upstream, downstream and intermediatetapping points, in which a first differential pressure monitoringarrangement is provided to measure the pressure differential between theupstream and the downstream tapping points and from which fluid densitycan be derived, and a second differential pressure monitoringarrangement is provided to monitor the pressure differential between theintermediate tapping point and one of the upstream or downstream tappingpoints, whereby flow rate can be calculated from the derived densityvalue and the second differential pressure measurement.

The invention therefore applies a fundamentally different method ofmonitoring the pressure differentials between the upstream, downstreamand the intermediate tapping points, which has a number of technicaladvantages over the known flow meter arrangement. In particular, theflow meter will usually be provided in a dedicated tubular section to beincorporated in an oil production well, and the novel arrangement of thefirst and second differential pressure monitoring arrangements enablethe overall length of the dedicated tubular section to be reducedcompared to existing arrangements.

In a first embodiment, the upstream tapping point is arranged in theregion of the upstream end of the transition pipe section, theintermediate tapping is arranged at any convenient position along thelength of the second pipe section, and the downstream tapping isarranged at a convenient position along the length of a fourth pipesection arranged downstream of the second pipe section. The densitycalculation is derived from the pressure differential monitored betweenthe upstream and downstream pressure tapping points, whereas the seconddifferential pressure measurement is made between the intermediatepressure tapping point and the downstream pressure tapping point.

In a second embodiment, the upstream and downstream pressure tappingpoints are arranged in generally similar position to the firstembodiment, but the second differential pressure measurement is madebetween the intermediate pressure tapping point and the upstreampressure tapping point.

In a third embodiment, the upstream pressure tapping point is arrangedin the first pipe section at a position spaced upstream of thetransition pipe section, whereas the downstream pressure tapping pointis arranged within the second pipe section, and the pressuredifferential measured between the upstream and downstream tapping pointsserves to enable the density to be derived. The intermediate pressuretapping point is arranged in the region of the upstream end of thetransition pipe section, and the second differential pressuremeasurement is made between the intermediate pressure tapping point andthe downstream pressure tapping point.

The diameter of the second pipe section is smaller than the diameter ofthe first pipe section, and is arranged downstream thereof, and thefourth pipe section, i.e. further downstream pipe section may have thesame diameter as the first pipe section, or may be different therefrom.

The embodiments of the invention enable individual flow rates of a twophase fluid to be measured e.g. the individual flow rates of oil andwater in an oil production well.

Fluid density is calculated by measuring the pressure difference betweentwo tapping points which are vertically spaced from each other, thoughthis vertical spacing need not be one directly above the other, as theflow meter of the invention may be employed both in a vertical or aninclined well. Gross flow rate is derived from a venturi device, andindividual oil and water flow rates can be easily calculated since oiland water densities will be known.

In the invention, the first differential pressure measurement (fromwhich density is derived) is made between the two extreme pressuretapping points of the three pressure tappings i.e. the upstream anddownstream tappings, and the advantage of this is that it maximises thedistance between the two measuring points which improves the accuracy ofthe density measurement. A further benefit is the reduced overall lengthof the device.

The measurement of the first and the second differential pressures maybe carried out using absolute pressure gauges at each pressure tappingpoint, or by use of differential gauges to measure the pressuredifferences between the appropriate pressure tapping points.

Three embodiments of flowmeter according to the invention will now bedescribed in detail, by way of example only, with reference to theaccompanying schematic drawing.

Referring now to the drawing, this shows a dedicated tubular flowmonitoring tool which can be incorporated in an oil production well, andwhich is designated generally by reference 10. The tool 10 comprises afirst pipe section 11 having a first diameter D1, a second pipe section12 of smaller diameter D2, and an internally tapering third pipe section13 which forms a smooth transition between first pipe section 11 andsmaller diameter second pipe section 12.

Three embodiments of the invention are shown schematically in the lowerpart of the drawing, designated by references A, B and C. In eachembodiment, there are three pressure tapping points comprising upstream,downstream and intermediate tapping points, and a first differentialpressure monitoring arrangement is provided to measure the pressuredifferential between the upstream and the downstream tapping points, andfrom which fluid density is derived, and a second differential pressuremonitoring arrangement is provided to monitor the pressure differentialbetween the intermediate tapping point and one of the upstream ordownstream tapping points. Fluid flow rate can then be calculated fromthe density value and from the second differential pressure measurement.

In the first embodiment, shown by reference A, the upstream pressuretapping point is designated by reference At_(u), the downstream tappingpoint by reference At_(d) and the intermediate tapping point byreference At_(i). It will be seen that the upstream pressure tappingpoint At_(u) is arranged in the region of the upstream end of thetapering third pipe section 13, whereas the downstream pressure tappingAt_(d) is arranged in a downstream fourth pipe section 14, whichpreferably, though not essentially, has an internal diameter D4 which isapproximately the same as diameter D1 of first pipe section 11. Thedensity calculation is derived from the differential pressure measuredbetween the upstream and the downstream pressure tapping point At_(u)and At_(d), whereas the second differential pressure measurement is madebetween the intermediate pressure tapping point At_(i) and thedownstream pressure tapping point At_(d).

In the embodiment shown by reference B, the upstream, downstream andintermediate pressure tapping points are located at the same positionsas for the embodiment A, and are designated by references Bt_(u), Bt_(d)and Bt_(i) respectively. However, while the first pressure differentialmeasurement from which the density is derived is made between theupstream and downstream pressure tapping points Bt_(u) and Bt_(d), inthis embodiment the second differential pressure measurement is madebetween the intermediate pressure tapping point Bt_(i) and the upstreampressure tapping point Bt_(u).

Finally, in the embodiment shown by reference C, the upstream,downstream and intermediate pressure tapping points are designated byreferences Ct_(u), Ct_(d) and Ct_(i). The first differential pressuremeasurement is made between the upstream and the downstream pressuretapping points Ct_(u) and Ct_(d), from which density is derived, but thesecond differential pressure measurement is made between theintermediate pressure tapping point Ct_(i) and the downstream pressuretapping point Ct_(d). However, in this embodiment, the upstream pressuretapping point Ct_(u) is arranged in the first pipe section 1, wellupstream of the third transition pipe section 13, the downstreampressure tapping point Ct_(d) is arranged within the second pipe section12, whereas the intermediate pressure tapping point Ct_(i) is arrangedin the region of the upstream end of the third transition pipe section13.

The differential pressure measurements may be made either using absolutepressure gauges at each pressure tapping point (shown schematically byreferences 1, 2, 3 and 4), or by using differential pressure gauges tomeasure the pressure differences between the appropriate pressuretapping points.

We claim:
 1. A flowmeter for measuring a flow rate of a two phase fluidin a pipe and which comprises a first pipe section having a firstdiameter, a second pipe section of a smaller diameter, an internallytapering transition pipe section forming a smooth transition between thefirst and second pipe sections, and three pressure tapping pointscomprising upstream, downstream and intermediate tapping points, whereinthe upstream and downstream tapping points are vertically spaced, inwhich a first differential pressure monitoring arrangement is providedto measure the pressure differential between the upstream and thedownstream tapping points and from which fluid density is derived, and asecond differential pressure monitoring arrangement is provided tomonitor the pressure differential between the intermediate tapping pointand one of the upstream and downstream tapping points, whereby flowrates of both phases of the two phase fluid is calculated from thederived density value and the second differential pressure measurement.2. A flowmeter according to claim 1, wherein the flowmeter is providedin a dedicated tubular section to be incorporated in an oil productionwell.
 3. A flowmeter according to claim 2, in which the upstream tappingpoint is arranged at a region of the upstream end of the transition pipesection, the intermediate tapping is arranged along the length of thesecond pipe section, and the downstream tapping is arranged along thelength of a fourth pipe section arranged downstream of the second pipesection.
 4. A flowmeter according to claim 3, wherein the densitycalculation can be derived from the pressure differential monitoredbetween the upstream and downstream pressure tapping points, and whereinthe second differential pressure measurement is made between theintermediate pressure tapping point and the downstream pressure tappingpoint.
 5. A flowmeter according to claim 3, wherein the densitycalculation can be derived for the pressure differential monitoredbetween the upstream and downstream pressure tapping points, and whereinthe second differential pressure measurement is made between theintermediate pressure tapping point and the upstream pressure tappingpoint.
 6. A flowmeter according to claim 2, in which the upstreamtapping point is arranged in the first pipe section at a position spacedupstream of the transition pipe section, the downstream pressure tappingpoint is arranged within the second pipe section and the intermediatepressure tapping point is arranged in the region of the upstream end ofthe transition pipe section.
 7. A flowmeter according to claim 6,wherein the pressure differential can be measured between the upstreamand downstream tapping points to enable the density to be derived, andthe second differential pressure measurement is made between theintermediate pressure tapping point and the downstream pressure tappingpoint.
 8. A flowmeter according to claim 1, in which the upstreamtapping point is arranged at a region of the upstream end of thetransition pipe section, the intermediate tapping is arranged along thelength of the second pipe section, and the downstream tapping isarranged along the length of a fourth pipe section arranged downstreamof the second pipe section.
 9. A flowmeter according to claim 8, whereinthe density calculation can be derived from the pressure differentialmonitored between the upstream and downstream pressure tapping points,and wherein the second differential pressure measurement is made betweenthe intermediate pressure tapping point and the downstream pressuretapping point.
 10. A flowmeter according to claim 8, wherein the densitycalculation can be derived from the pressure differential monitoredbetween the upstream and downstream pressure tapping points, and whereinthe second differential pressure measurement is made between theintermediate pressure tapping point and the upstream pressure tappingpoint.
 11. A flowmeter according to claim 8, in which fluid density iscalculated by measuring the pressure difference between two tappingpoints which are vertically spaced from each other.
 12. A flowmeteraccording to claim 1, in which the upstream tapping point is arranged inthe first pipe section at a position spaced upstream of the transitionpipe section, the downstream pressure tapping point is arranged withinthe second pipe section and the intermediate pressure tapping point isarranged in the region of the upstream end of the transition pipesection.
 13. A flowmeter according to claim 12, wherein the pressuredifferential can be measured between the upstream and downstream tappingpoints to enable the density to be derived, and the second differentialpressure measurement is made between the intermediate pressure tappingpoint and the downstream pressure tapping point.
 14. A flowmeteraccording to claim 13, in which fluid density is calculated by measuringthe pressure difference between two tapping points which are verticallyspaced from each other.
 15. A flowmeter according to claim 14, whereinthe flowmeter is oriented vertically for use in a vertical well, inwhich the two tapping points from which fluid density is derived arearranged directly one above the other.
 16. A flowmeter according toclaim 14, wherein the flowmeter is inclined for use in an inclined well,in which the two tapping points from which fluid density is derived arevertically spaced from each other, but not directly one above the other.17. A flowmeter according to claim 12, in which fluid density iscalculated by measuring the pressure difference between two tappingpoints which are vertically spaced from each other.
 18. A flowmeteraccording to claim 1, in which fluid density is calculated by measuringthe pressure difference between two tapping points which are verticallyspaced from each other.
 19. A flowmeter according to claim 18, whereinthe flowmeter is oriented vertically for use in a vertical well, inwhich the two tapping points from which fluid density is derived arearranged directly one above the other.
 20. A flowmeter according toclaim 18, wherein the flowmeter is inclined for use in an inclined well,in which the two tapping points from which fluid density is derived arevertically spaced from each other, but not directly one above the other.